1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed on the support, which is used in an analog or digital copying machine, laser printer and laser facsimile apparatus.
2. Discussion of Background
Inorganic materials such as selenium, cadmium sulfide and zinc oxide are used as photoconductive materials in conventional electrophotographic photoconductors. While the above-mentioned inorganic materials have shortcomings in toxicity, heat-resistance and manufacturing cost, the electrophotographic photoconductors employing a variety of organic photoconductive materials can be manufactured at low cost by mass production without environmental pollution. Therefore, studies of the organic electrophotographic photoconductor have been made with the aforementioned merits taken into account. Further, a function-separating photoconductor comprising a charge generation layer and a charge transport layer is proposed to obtain high photosensitivity and high durability, and some of the function-separating photoconductors have been put to practical use.
The organic photoconductor is generally fabricated in such a manner that a charge generating material, a charge transporting material and a binder resin are dissolved or dispersed in an organic solvent to prepare a coating liquid for the photoconductive layer, and the thus prepared coating liquid is coated on an electroconductive support and dried under application of heat thereto. Thus, a photoconductive layer is formed on the electroconductive support. However, the residual stress is generated in the photoconductive layer at the drying step in the formation of the photoconductive layer, which causes the curling and peeling problems of the photoconductive layer.
To decrease the residual stress generated in the photoconductive layer, it is proposed to add to a coating liquid for the photoconductive layer a variety of commercially available plasticizers such as alkyl phthalate (as disclosed in Japanese Laid-Open Patent Application 1-134364); o-terphenyl and m-terphenyl (as disclosed in Japanese Laid-Open Patent Application 3-134670); and a biphenyl derivative (as disclosed in Japanese Laid-Open Patent Application 3-75754).
However, the addition of the plasticizer to the photoconductive layer causes some problems in the photoconductive characteristics of the photoconductive layer, for instance, a decrease in photosensitivity and a change in photosensitivity during the repeated operation, especially under the circumstances of high temperature and humidity.
There is no electrophotographic photoconductor capable of meeting the requirements for both the plasticity and the photoconductive characteristics of the photoconductive layer.